Nunez said seven of the 13 were purchased either by the shooter or relatives in the past three years, and the guns were not all bought locally.

The Douglas County Sheriff's office clarified via Twitter that two pistols, four rifles and one shotgun were found at his house, while five different pistols and another rifle were found at the school.

Investigators also found a flak jacket lying next to the rifle at the school that had steel plates inside and five ammunition magazines, officials said.

The shooter engaged with police when they responded to emergency calls for help at the school, and he died during the exchange, though it is unclear whether he was killed by police or by his own hand.

Sheriff Hanlin has reiterated that his office will not publicly identify the shooter out of respect for the victims, though the U.S. Army has confirmed that the shooter went through five weeks of Army basic training at Fort Jackson in South Carolina in 2008.

He was discharged in 2008 "for failing to meet the minimum administrative standards to serve in the U.S. Army," a spokesman for the Army told ABC News, based on the military's records.

Ian Mercer, the suspected shooter’s father, briefly spoke to reporters outside of his home Thursday, calling the situation a “devastating day for me and my family.”